1. Technical Field
The present disclosure relates to a semiconductor device and a fabrication method thereof, and more particularly, to a not-and (NAND) flash memory device and a fabrication method thereof.
2. Description of the Related Art
A flash memory device is a kind of non-volatile memory devices maintaining stored information regardless of whether or not power is supplied to the device. Unlike a read only memory (ROM), which is another kind of a non-volatile memory device, the flash memory device has a characteristic of electrically changing stored information rapidly and easily.
The flash memory device can be classified into, for example, a NOR-type flash memory device and a NAND-type flash memory device depending upon the structure in which memory cells are connected to bit lines and ground electrodes. For example, the NOR-type flash memory device (referred to as a NOR flash memory) has a structure in which memory cells are connected in parallel to one another between the bit lines and the ground electrodes, achieving rapid random access. Therefore, the NOR flash are widely used for example, for basic input output system (BIOS), cellular phones, and personal digital assistants (PDAs).
On the other hand, the NAND-type flash memory device (referred to as a NAND flash memory) has a structure in which memory cells are connected in series between bit lines and ground electrodes (refer to FIG. 1). For example, a cell array 100 of the NAND flash memory includes a plurality of cell strings 10, and each cell string 10 includes a plurality of memory cells that are connected in series. At this point, ground selection transistors connected by ground selection lines GSL and string selection transistors connected by string selection lines SSL are arranged at both ends of the cell strings 10, to control the electrical connection between the memory cells and the bit lines 40/ground electrodes 45.
The NAND flash memory device has the highest integration among available semiconductor devices due to its series connection structure. Also, as the NAND flash memory device uses an operational method of simultaneously changing information stored in a plurality of memory cells, the information updating speed of the NAND flash memory device is significantly greater than that of the NOR flash memory device. With such a high integration and rapid updating speed, the NAND flash memory device is widely used for portable electronic products that require mass storage such as, for example, digital cameras and MP3 players. Moreover, it is expected that the market for NAND flash products will expand even further due to the rapidly increasing demand for portable electronic products.
As well known in the art, NAND flash memory devices update information stored in selected memory cells using Fowler-Nordheim tunneling, and this information updating operation (e.g., programming and erasing operations) is simultaneously performed on a page or block consisting of a plurality of memory cells as described above. At this point, a programming operation by a page unit may cause a difficulty in that an unselected memory cell (particularly, a memory cell adjacent to a selected memory cell) located along a selected word line WL may be programmed. This inadvertent programming of an unselected memory cell is generally called “program disturbance”.
To prevent such program disturbance, the programming operation of the NAND flash memory is performed using self-boosting scheme, which is described in U.S. Pat. No. 5,677,873 titled “METHOD OF PROGRAMMING FLASH EEPROM INTEGRATED CIRCUIT MEMORY DEVICES TO PREVENT INADVERTENT PROGRAMMING OF NONDESIGNATED NAND MEMORY CELLS THEREIN” and U.S. Pat. No. 5,991,202 titled “METHOD FOR REDUCING PROGRAM DISTURB DURING SELF-BOOSTING IN A NAND FLASH MEMORY”.
The above-mentioned self-boosting scheme controls the voltage applied to gate electrodes of string selection lines SSL and ground selection lines GSL such that an unselected cell string is not electrically connected to a bit line 40 and a ground electrode 45 connected to the unselected cell string. When the cell string 10 is electrically isolated, the voltage applied to a portion of a substrate 1 located under unselected memory cells is raised up to a predetermined voltage by a program voltage applied to a selected word line WL (refer to FIG. 2). The above-mentioned voltage rise is called boosting. Moreover, when the voltage of the substrate is raised, the potential difference between a floating gate electrode 22 and the substrate 1 is reduced, thereby cutting off Fowler-Nordheim tunneling through a tunnel insulation layer 70. Consequently, the above-described program disturbance is prevented.
With the above-described mechanism of self-boosting scheme, an unselected cell string should be completely isolated from the bit line 40 and the ground electrode 45 to prevent the program disturbance. In addition, to achieve the electrical isolation of the cell string 10, the turn-off current characteristic for the string selection transistor and the ground selection transistor should be improved. However, according to a conventional art, as the gate insulation layer 75 of the selection transistors is formed with the same thickness as the thickness of the tunnel insulation layer 70 of memory cell transistors as illustrated in FIG. 2, there thus may be limitations in being able to improve the turn-off current characteristics of selection transistors. For example, with conventional NAND flash memory devices, reducing the linewidth of the selection transistor for increasing integration may cause a short-channel effect, thereby reducing the turn-off current characteristic of the selection transistor.
Thus, there is a need for NAND flash memory devices which prevent program disturbance and which include selection transistors having improved turn-off current characteristics and for methods of fabricating the same.